Two possibilities here: either fuel seepage from the inlet (remove, clean, assemble with new gasket) or from the air horn gasket.
If its the latter: Maybe the front bolts got a bit lose over time and need a little retightening. Maybe the gasket is bad and needs replacement, and maybe even the castings are slightly warped.
In case tightening doesn´t show any effect, remove fuel line (have a glass or other small container and some rags handy for gas spillage) and unbolt all of the top bolts holdting the upper part (so-called air horn assembly), then carefully lift it upwards. The remains of the gasket might separate. Take care not to bend or break the (mucho useless) plastic vent rod that connects to the acc.pump lever. Scrape or file off the stake markings at the vent rod opening and carefully remove the vent rod and spring assembly.
This can be done with the carburetter still in place, or removed for further rebuild.
lower casting warpage can be checked with a straight metal ruler and feeler gauges; I´ll have to look up the tolerances yet - more later.
If the carb hasn´t been rebuilt yet, I´d suggest getting a carb repair kit (hygrade and KEM are good quality brands) and renew the whole carb. This ain´t no rocket science and can be accomplished in one afternoon (or two or three, depending how fuzzy you get about details).
If the carb turns out to be super straight with just a bad gasket, carefully remove any gasket remains off the casting flanges, put on a new gasket and put everything back together.
Drop me a line in case you just need a single gasket. I have a couple spares.